"We have demonstrated a strategy for intentionally positioning molecules, which is necessary for the construction of nanoscale systems such as molecular transistors or light-emitting diodes," said Mark Hersam of Northwestern.
"Our process works at room temperature and on silicon, which suggests that it can be made compatible with conventional silicon microelectronics," he added. "Ultimately we want to integrate with current technology, creating a bridge between microelectronics and nanoelectronics."
Hersam and colleagues used the STM's localized electron beam to desorb two individual hydrogen atoms from the same row of a hydrogen-terminated Si(100) surface. This process is known as feedback-controlled lithography.
Then the scientists introduced 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radicals. These attached to the dangling bonds left behind by the desorbed hydrogen atoms.
To date it's been hard to make nanostructures from multiple species in this way. That's because carrying out feedback-controlled lithography a second time next to the original location can cause the newly adsorbed species to desorb. The Northwestern technique avoids the problem by combining feedback-controlled lithography with the spontaneous growth of a styrene chain.
To achieve this, the team used feedback-controlled lithography to create another dangling bond at a location in between the two TEMPO molecules. The styrene's carbon-carbon double bond reacted with this dangling bond. The result was a carbon radical on the styrene molecule that removed a hydrogen atom from a neighbouring silicon dimer. In turn, this left a new dangling bond to react with another styrene molecule.
Growth continued until the chain reached the TEMPO molecules. This produced a nanostructure on the silicon surface that contained different types of molecules - the first time this has been achieved at room temperature.
"Previously we were working with single molecules on silicon," said Hersam. "This new process enables us to build more complex structures. Plus, the technique is general and can be used with many different molecules, which increases its potential."
The researchers reported their work in Applied Physics Letters.